West Nile Virus: Questions and Answers

July 30, 2012 -- How extensive is the threat of West Nile Virus this summer, and how can you help protect yourself from it? WebMD asked the CDC's medical epidemiologist Marc Fischer, MD.

What is the status of West Nile so far this summer?

Well, so far this year, some areas of the country are experiencing substantially more activity than they have in recent years.

As of July 24, when we pulled our last report, 35 states in the U.S. have reported West Nile virus infections in people, birds, or mosquitoes. To date, there have been 113 human cases of West Nile virus disease [this season], including two deaths, and most of these cases have been reported from South-Central states, which include Texas, Mississippi, Oklahoma, and Louisiana.

The overall number of cases is the highest number that we've seen reported through the third week of July since 2007.

Some areas experience more activity from year to year, but it's very difficult to predict how severe this season will be or why seasons are different from one year to another or from area to area.

Many factors impact when and where outbreaks occur, including:

Weather

The number of mosquitoes that spread the virus

The number of birds that harbor the virus

Human behavior that allows them to be exposed to the virus

What kind of season are you expecting, regarding the virus? How has the weather affected that?

It's really difficult to make a prediction for the severity of the season, so it is important that people be aware of that [West Nile virus] activity and take steps to protect themselves.

Because things can change over the year, it can be that the year's activity is going to be larger than in recent years, or it can be that it will be an earlier season, and so we'll end up having cases [now] and then smaller numbers later.

But typically, the season nationally peaks in the first few weeks of August, which is the time we're just moving into, and given the amount of activity we're seeing so far this early, it is important that people protect themselves against West Nile virus.

Weather, we know, plays a role in the amount of activity, but [determining] the specifics of what weather and what drives the amount of activity in cases is very difficult. Certainly temperature and precipitation play roles, but it's a more complex relationship to what's going to happen regarding activity.